A modulation method called an Orthogonal Frequency Division Multiplex (OFDM) method is adopted for digital terrestrial broadcasting such as Integrated Services Digital Broadcasting-Terrestrial (ISDB-T).
With this orthogonal frequency division multiplex method, as shown in FIG. 6, one OFDM symbol period is divided into an effective symbol section and a guard interval section in order to transfer a digital signal. The guard interval section is a redundant signal section copying aback portion of the effective symbol section, and is disposed in front of the effective symbol section to show the same waveform as the back portion of the effective symbol section at a symbol cycle.
FIG. 7 is a diagram showing an example of the structure of a conventional reception apparatus for receiving a broadcast signal in a system using the orthogonal frequency division multiplexed method.
With this structure, a guard correlation processor 56 performs a auto-correlation calculation of a received signal so that the position of a guard interval section can be detected. In accordance with a result of the auto-correlation calculation performed by the guard correlation processor 56, a timing adjustment unit 57 generates a control voltage for adjusting a reception timing and supplies it to a frequency variable oscillator such as a Voltage Controlled crystal Oscillator (VCXO) 59 via a loop filter 58.
During mobile reception of digital terrestrial broadcasting, the level of a reception signal of a direct wave synchronized with a reception timing may be lowered by the influence of an obstacle such as a building and there is a possibility that a path for direct wave propagation may be almost lost. If such an obstacle does not exist, there is a possibility that a direct wave propagating a faster path may arrive.
If a Single Frequency Network (SFN) is configured on the transmission side, radio waves transmitted from a plurality of transmission antennas arrive at the reception side via various paths so that the path to which the reception timing is set may disappear or a faster path may be formed abruptly.
For mobile reception of digital terrestrial broadcasting with a conventional reception apparatus, the gain of the loop filter 58 shown in FIG. 7 is set larger than that for broadcasting reception at a fixed position, in order to speed up a response speed of VCXO 59.
In this case, if a faster path than the presently set reception timing appears, the reception timing is quickly adjusted so as to receive a radio wave propagating the faster path. If a path with an adjusted reception timing disappears, the reception timing is quickly adjusted so as to receive a radio wave propagating the slower path. In this manner, a radio wave propagating a much slower path can be received.
If a path faster than the presently set reception timing appears, this path exceeds the guard interval section. In this case, the reception timing is required to be quickly adjusted. If transmission data is demodulated in the state that adjacent OFDM symbols (effective symbols) are mixed, inter-symbol interference is generated and the demodulated waveform is not good.